The Digestive System Chapter 16

Overview The digestive system functions to break down food into a usable form for the body Uses include energy for cellular metabolism and growth (you are what you eat) Nutrients can be extracted from nearly any food and the molecular building blocks are then rearranged according to the body’s needs.

Divisions of the Digestive System The digestive system is divided into two basic parts; the alimentary canal and accessory organs The alimentary canal (also the gastrointestinal tract or GI tract) is a hollow tube extending from the opening of the mouth to the anus Accessory organs are located within or outside the alimentary canal and assist with functions of the organs of the alimentary canal

Parts of the Digestive System Alimentary Canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine Accessory Organs – teeth, tongue, salivary glands, pancreas, liver, and gallbladder

Digestive Processes Ingestion – bringing food in the digestive system Mechanical digestion – breaking food into smaller pieces and mixing it up Propulsion – movement of food through alimentary canal; includes swallowing and peristalsis

Digestive Processes Chemical digestion – breakdown of large molecules of food into their basic chemical building blocks which are small enough to be absorbed Absorption – the diffusion of food into the bloodstream from the alimentary canal Defecation – the elimination of indigestible material from the body in the form of feces

Wall Structure of Alimentary Canal Each part of the alimentary canal has the same 4 layers, though the cellular structure of each layer in different parts may differ slightly Mucosa – deepest layer, mucous membrane; protects from microorganisms, absorbs digested food, and secrete mucous and digestive enzymes

Wall Structure of Alimentary Canal Submucosa – rich in blood vessels, lymphatic vessels, nerve endings, and small glands; blood vessels carry away absorbed materials Muscularis – consists of smooth muscle in two layers (longitudinal and circular) forming sphincters at several points; mixes and propels food

Wall Structure of Alimentary Canal Serosa – covers the alimentary canal and secretes serous fluid to lubricate movements between its parts

The Mouth The mouth receives food and prepares it for swallowing by mechanically breaking it down and mixing it with saliva The lips are rich in blood vessels and nerve endings and help keep food in the mouth while chewing The tongue helps to mix food with saliva while chewing and moves it to the back of the throat while for swallowing; covered with taste buds which provide information about food to the brain

Dentition The teeth of the mouth are of two different types Deciduous teeth (baby teeth) develop first and fall out as permanent teeth grow beneath them; all 20 are usually present by about 24 months of age Permanent teeth (adult teeth) emerge throughout late childhood and adolescence except for the wisdom teeth which may not erupt until the mid-twenties

Types of Teeth Incisors – chisel shape helps to cut food Canines – conical shape helps to pierce and tear food Premolars and molars – help to grind and crush food

Salivary Glands Secrete saliva, which is mostly water (99.5%) with some solutes, mostly mucous and enzymes Necessary for gustation (tasting) Binds and lubricates food for swallowing forming a bolus Begins the breakdown of carbohydrates and kills many bacteria

Salivary Glands Largest is the parotid gland located in front of and below each ear Submandibular gland is located along the inner surface of the jaw Sublingual gland is located in front of the submandibular gland and below the tongue

Pharynx Transports food from the oral cavity to the esophagus The epiglottis closes to keep food out of the trachea

Esophagus Extends approx. 10 inches to the stomach The esophageal sphincter controls the flow of food into the stomach and keeps it in there If this sphincter is weak it can permit the backflow of chyme from the stomach to produce heartburn

Stomach Highly extensible folded bag with an extra layer of oblique muscle fibers in the muscularis Mixes bolus with HCl, pepsinogin, pepsin (breaks down most proteins), and intrinsic factor which enables the absorption of vitamin B 12 Some mechanical and major chemical digestion take place here A limited amount of absorption also takes place Entrance to the small intestine is controlled by the pyloric sphincter

Pancreas Produces about a 1.5 liters of pancreatic juice daily which enters the duodenum via the common bile duct Pancreatic juice contains powerful enzymes which can break down carbohydrates, proteins, and fats

Liver The liver, among its other functions, produces bile for the emulsification of fats This is stored in the gall bladder then released into the duodenum by way of the common bile duct

Sheep’s Liver

Small intestine Extends almost 20 feet, divided into three sections – the duodenum, jejunum, and ileum The mucosa is highly folded and has fingerlike projections called villi which intrude approx. 1 mm into the lumen The villi are also studded with microvilli which dramatically increase surface area for absorption

Large Intestine Extends for about 5 feet Extracts water from indigestible food, eventually forming feces Feces is made up of indigestible food (fiber), mucous, old cells from the alimentary canal lining, and bacteria that reside within

How Many Structures Can You Identify?